Add debug logs wherever we change current logcontext (`LoggingContext`).
I've had to make this same set of changes over and over as I've been
debugging things so it seems useful enough to include by default.
Instead of tracing things at the `set_current_context(...)` level, I've
added the debug logging on all of the utilities that utilize
`set_current_context(...)`. It's much easier to reason about the log
context changing because of `PreserveLoggingContext` changing things
than an opaque `set_current_context(...)` call.
Spawning from
https://github.com/matrix-org/synapse/pull/12588#discussion_r865843321
> It turns out `Deferred.cancel()` is a lot like
`Deferred.callback()`/`errback()` in that it will trash the logging
context:
> it can resume a coroutine, which will restore its own logging context,
then run:
>
> - until it blocks, setting the sentinel context
> - or until it terminates, setting the context it was started with
>
> So we need to wrap it in `with PreserveLoggingContext():`, like we do
with `.callback()`:
>
> ```python
> with PreserveLoggingContext():
> self.render_deferred.cancel()
> ```
>
> *-- @squahtx,
https://github.com/matrix-org/synapse/pull/12588#discussion_r865843321*
Also adds a section in the docs explaining the `sentinel` logcontext.
Spawning from https://github.com/element-hq/synapse/pull/18870
### Testing strategy
1. Run Synapse normally and with `daemonize: true`: `poetry run
synapse_homeserver --config-path homeserver.yaml`
1. Execute some requests
1. Shutdown the server
1. Look for any bad log entries in your homeserver logs:
- `Expected logging context sentinel but found main`
- `Expected logging context main was lost`
- `Expected previous context`
- `utime went backwards!`/`stime went backwards!`
- `Called stop on logcontext POST-0 without recording a start rusage`
- `Background process re-entered without a proc`
Twisted trial tests:
1. Run full Twisted trial test suite.
1. Check the logs for `Test starting with non-sentinel logging context ...`
* Pull Sentinel out of LoggingContext
... and drop a few unnecessary references to it
* Factor out LoggingContext.current_context
move `current_context` and `set_context` out to top-level functions.
Mostly this means that I can more easily trace what's actually referring to
LoggingContext, but I think it's generally neater.
* move copy-to-parent into `stop`
this really just makes `start` and `stop` more symetric. It also means that it
behaves correctly if you manually `set_log_context` rather than using the
context manager.
* Replace `LoggingContext.alive` with `finished`
Turn `alive` into `finished` and make it a bit better defined.
